1. Field of the Invention
The present invention relates to a friction engagement device of an automatic transmission. More specifically, the present invention relates to a friction engagement device used as a brake or a clutch of an automatic transmission.
2. Description of the Background Art
Japanese Patent Laying-Open No. 11-141662, for example, discloses a conventional friction engagement device of an automatic transmission which ensures a rigidity of a brake piston without causing interference with a car body floor. The automatic transmission disclosed in this publication includes friction discs on a fixed side and a rotation side which form a multiple disc brake, and a ring-like brake piston for pressing the friction discs. An oil pressure chamber is formed in a position adjacent to the brake piston, and an oil hole connecting to an oil pressure control valve unit is formed in a bottom portion of the oil pressure chamber.
In the automatic transmission disclosed in Japanese Patent Laying-Open No. 11-141662, an oil pressure is supplied from the oil pressure control valve unit to the oil pressure chamber through the oil hole. With this, the brake piston strokes and allows friction discs of the fixed side and the rotation side to engage with each other. Since the oil hole is provided only in the bottom portion of the oil pressure chamber, however, the brake piston may press the friction discs in an inclined state. In this situation, a load to the friction discs becomes uneven, which degrades durability of the friction discs. In addition, a force applied to the friction disc varies in a circumference of the friction disc, which disturbs a smooth speed-changing operation. Such phenomenon occurs more frequently at a lower temperature with an increased viscosity of oil, and therefore reliability of the automatic transmission decreases particularly when it is used in cold climates.